Abstract: A method that alternating sine current superposes on direct current was adopted for study on alternating current impedance of the small current TIG arc.It was found that with the frequency getting high,the modulus of arc impedance increased and then decreased,and the phase angle decreased continuously.The maximum of the modulus just coincided with the zero point of phase angle,and obviously,arc impedance had a resonance.By analyzing the voltage and current data in many different frequencies,the electronic model and transfer function of TIG arc alternative component were deduced.The parameters of transfer function were fit by using mathematics software,and the resonance frequency was calculated.